If the weather had been better, we could have had a picnic. But it all day.A. rainedB. ra

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(答案→)A. rained
B. rains
C. has rained
D. is raining
解析：从题干前部分句子的谓语动词结构（虚拟语气）可以看出是指过去某个时间已经下了整天的雨。所以应该用一般过去时态。该题解题关键要从题干前部分入手。if非真实条件句用的过去完成时，主句用would have done，我们可以看出这是与过去事实相反的虚拟语气。